Since colonisation, a dizzying array of Australia’s native species and ecosystems have been altered or removed altogether. It therefore seems natural to consider the idea of restoring what’s been lost – a process termed “rewilding”.

Now a global trend, rewilding projects aim to restore functional ecosystems. The rationale is that by reactivating the often complex relationships between species – such as apex predators and their prey, for example – these ecosystems once again become able to sustain themselves.

Rewilding has successfully captured the public interest, particularly overseas. Conservation group Rewilding Europe has a network of eight rewilding areas and a further 59 related projects, covering 6 million hectares in total.

The reintroduction of wolves to Yellowstone National Park in the United States remains the most recognised example of rewilding. The wolves reduced elk numbers and changed their behaviour, which allowed vegetation to grow and stabilise stream banks.

But in Australia, we need to do rewilding differently. The particular challenges we face with issues such as introduced species mean that, like Vegemite, our rewilding future must have a unique flavour.

Australian values

Our recently published paper builds on findings from a rewilding forum held in Sydney in late 2016. Academics, government and non-government agencies met to discuss some of the outstanding issues around rewilding in Australia. Despite the large, diverse audience and wide-ranging views, the forum succeeded in identifying some key themes.

A much bigger Peninsula, Yorke, in South Australia, is the site of an ambitious project to reintroduce 20 species currently extinct in the area.

The widespread removal of dingoes has reduced natural control on introduced species such as foxes and cats. This in turn has allowed feral cats and red foxes to prey on small digging native mammals in the absence of a larger competitor. Meanwhile, declines in Tasmanian devils have been mirrored by declines in smaller predators like eastern quolls, and changes to cat behaviour, suggesting devils indirectly affect other species.

Many of these digging mammals have declined continent-wide and disappeared completely from other areas. This in turn has resulted in knock-on effects, such as altered fire regimes and changes to plant diversity.

It’s no surprise, then, that our workshop identified restoring predators and small mammals as priorities in Australia. Lots of work is already going on to restore small mammal populations, such as via Australian Wildlife Conservancy’s and Arid Recovery’s fenced exclosures, from which foxes and cats have been eliminated.

But exclosures are also contrary to the aims of rewilding, because they need ongoing maintenance and do not help the ecosystem inside to be self-sustaining. They can also exacerbate prey naïveté, whereby the native mammals fail to recognise and avoid introduced predators.

It may therefore be useful to view fences as a stepping stone to restoring small mammal populations to broader landscapes, helped by a variety of means including promoting co-evolution of native and introduced species, incentives to farmers, and the use of guardian animals.

Australia is different

Passive rewilding – the removal of human agriculture, resulting in the return of natural vegetation – has had positive impacts on biodiversity in Europe. It could have similarly positive impacts here, for example by increasing the density of tree hollows in previously logged forest and woodland. But a complete removal of management is unlikely to be effective because of, for example, the need to manage fire and the presence of introduced species like miner birds that exert influences on other species and even entire ecosystems.

In arid areas, simply removing agriculture is unlikely to halt the declines in biodiversity unless deliberate steps are taken to control pest plants and animals and to shift the ecosystem into a preferred state. In oceans, where rewilding is no less urgent due to declines in large predatory fish, passive rewilding may be more feasible, as marine protected areas can result in recovery of fish, provided certain key criteria are met.

Reintroducing large (bigger than 100kg) herbivores is part of rewilding efforts in Europe and Asia. Yet in Australia, all large herbivores are introduced and are generally perceived as having negative impacts.

“Natural” control of such species is not possible due to a lack of big native predators. Introducing “surrogates” of long-extinct predators (as are used elsewhere in rewilding) would have predictable results in terms of human acceptance (farmers wouldn’t like it), but uncertain impacts on ecosystems.

There is clearly an appetite among scientists and managers for bold interventions such as trial reintroductions of Tasmanian devils to mainland Australia to restore ancient food chains or allowing dingoes to return to areas where they occur at low densities and are functionally extinct. Rewilding’s focus on restoration of ecosystem processes can complement, but not replace, existing conservation approaches. For example, we still need to achieve a comprehensive, adequate and representative reserve network on land and at sea.

Development of a rewilding vision and strategy would be a valuable first step towards maximising the potential of rewilding, tracking success, and persuading governments to fund it.

The authors would like to gratefully acknowledge the contribution of Dr Andy Sharp, Natural Resources Northern and Yorke Manager Planning and Programs, to this article.

Throughout history, humans have taken plants and animals with them as they travelled the world. Those that survived the journey to establish populations in the diaspora have found new opportunities as they integrate into new ecosystems.

These immigrant populations have come to be regarded as “invaders” and “aliens” that threaten pristine nature. But for many species, migration may just be a way to survive the global extinction crisis.

In our recently published study, we found that one of the Earth’s most imperilled group of species is hanging on in part thanks to introduced populations.

Megafauna – plant-eating terrestrial mammals weighing more than 100kg – have established in new and unexpected places. These “feral” populations are rewilding the world with unique and fascinating ecological functions that had been lost for thousands of years.

Today’s world of giants is only a shadow of its former glory. Around 50,000 years ago, giant kangaroos, rhino-like diprotodons, and other unimaginable animals were lost from Australia.

A wilder world than we think

Formal conservation distribution maps show that much of Earth is empty of megafauna. But this is only a part of the picture.

Many megafauna are now found outside their historic native ranges. In fact, thanks to introduced populations, regional megafauna species richness is substantially higher today than at any other time during the past 10,000 years.

Megafauna have expanded beyond their historic native range to rewild the world. Number of megafauna per region, in their ‘native’ range only (a) and in their full range (b)Modified and reproduced from Lundgren et al. 2017

Worldwide introductions have increased the number of megafauna by 11% in Africa and Asia, by 33% in Europe, by 57% in North America, by 62% in South America, and by 100% in Australia.

Australia lost all of its native megafauna tens of thousands of years ago, but today has eight introduced megafauna species, including the world’s only wild population of dromedary camels.

Australia lost all of its native megafauna tens of thousands of years ago, but is now home to eight introduced species, including the world’s only population of wild dromedary camels. Remote camera trap footage from our research program shows wild brumbies, wild donkeys and wild camels sharing water sources with Australian dingoes, emus and bustards in the deserts of South Australia.

These immigrant megafauna have found critical sanctuary. Overall, 64% of introduced megafauna species are either threatened, extinct, or declining in their native ranges.

Some megafauna have survived thanks to domestication and subsequent “feralisation”, forming a bridge between the wild pre-agricultural landscapes of the early Holocene almost 10,000 years ago, to the wild post-industrial ecosystems of the Anthropocene today.

Wild cattle, for example, are descendants of the extinct aurochs. Meanwhile, the wild camels of Australia have brought back a species extinct in the wild for thousands of years. Likewise, the vast majority of the world’s wild horses and wild donkeys are feral.

There have been global calls to rewild the world, but rewilding has already been happening, often with little intention and in unexpected ways.

A small population of wild hippopotamuses has recently established in South America. The nicknamed “cocaine hippos” are the offspring of animals who escaped the abandoned hacienda of Colombian drug lord Pablo Escobar.

By insisting that only idealised pre-human ecosystems are worth conserving, we overlook the fact that these emerging new forms of wilderness are not only common but critical to the survival of many existing ecosystems.

Vital functions

Megafauna are Earth’s tree-breakers, wood-eaters, hole-diggers, trailblazers, wallowers, nutrient-movers, and seed-carriers. By consuming coarse, fibrous plant matter they drive nutrient cycles that enrich soils, restructure plant communities, and help other species to survive.

The wide wanderings of megafauna move nutrients uphill that would otherwise wash downstream and into the oceans. These animals can be thought of as “nutrient pumps” that help maintain soil fertility. Megafauna also sustain communities of scavengers and predators.

In North America, we have found that introduced wild donkeys, locally known as “burros”, dig wells more than a metre deep to reach groundwater. At least 31 species use these wells, and in certain conditions they become nurseries for germinating trees.

The removal of donkeys and other introduced megafauna to protect desert springs in North America and Australia seems to have led to an exuberant growth of wetland vegetation that constricted open water habitat, dried some springs, and ultimately resulted in the extinction of native fish. Ironically, land managers now simulate megafauna by manually removing vegetation.

It is likely that introduced megafauna are doing much more that remains unknown because we have yet to accept these organisms as having ecological value.

Living in a feral world

Like any other species, the presence of megafauna benefits some species while challenging others. Introduced megafauna can put huge pressure on plant communities, but this is also true of native megafauna.

Whether we consider the ecological roles of introduced species like burros and brumbies as desirable or not depends primarily on our own values. But one thing is certain: no species operates in isolation.

Visions of protected dingoes hunting introduced donkeys and Sambar deer in Australia, or protected wolves hunting introduced Oryx and horses in the American West, can give us a new perspective on conserving both native and introduced species.